A permanent magnet buried in a rotor of an IPM motor or the like is required to have a coercive force so as to resist demagnetization due to external magnetic field entering from a stator core side.
Such external magnetic field acting on the permanent magnet typically becomes the largest at a corner of the permanent magnet on the stator core side in a plan view of the rotor with the permanent magnet buried therein, and becomes smaller on a center side of the rotor core.
Meanwhile, a sintered permanent magnet includes metal particles that are grain-boundary diffused from a surface thereof in order to improve coercive force performance of the permanent magnet, and these metal particles include rare earth such as dysprosium or terbium. Therefore, one of important issues to be solved is, from the viewpoint of a reduction in manufacturing cost of permanent magnets, how to reduce the usage amount of these metal particles while assuring desired coercive force performance.
Since the magnitude of external magnetic field acting on a permanent magnet varies from one part to another of the permanent magnet as stated above, a coercive force required also will vary from one part to another of the permanent magnet. With consideration further given to a reduction in usage amount of rare earth used to enhance the coercive force performance, a coercive-force distributed magnet with a different coercive force for each part of the permanent magnet (with distributed coercive force) is to be manufactured, whereby a permanent magnet can be manufactured so as to minimize the usage amount of rare earth such as dysprosium to allow for a reduction in the manufacturing cost while satisfying the required coercive force performance.
It is very important, from the viewpoint of quality assurance of a coercive-force distributed magnet, to precisely specify the internal coercive force distribution, i.e., an average coercive force for each internal part of a coercive-force distributed magnet. For instance, the coercive-force distributed magnet that is buried in the rotor for IPM motor as stated above may be optimally designed so that the magnetic characteristics of a side part on the stator side becomes relatively favorable, which is attributable to the flow of magnetic flux from the stator side. In that case, it is very important for the future development of a product (such as a magnet) as well as for a magnet maker and a maker using a magnet to precisely specify a coercive force for each internal part of a coercive-force distributed magnet at the stage before the magnet is brought into use and to guarantee the quality of the coercive-force distributed magnet as a specifying target for each desired part more precisely.
Currently, however, a method used is just to break a coercive-force distributed magnet into separated pieces and specify a coercive force thereof. Even when the coercive-force measurement methods disclosed in Patent Documents 1 and 2 as prior arts are used, an average coercive force of a coercive-force distributed magnet as a whole can be just measured, and an average coercive force for each part (for each divisional area) of the coercive-force distributed magnet cannot be specified.
In this way, instead of a conventional specifying method of breaking a coercive-force distributed magnet and specifying a coercive force for each part, an apparatus capable of creating a demagnetization curve for each divisional area obtained by dividing the magnet into any areas without breaking the magnet and specifying an average coercive force for each divisional area has been sought.
Patent Document 1: JP Patent Publication (Kokai) No: 2001-141701 A
Patent Document 2: JP Patent Publication (Kokai) No. 05-264704 A (1993)